An amplifier arrangement is used in many technical devices. An example of this is a pulse-echo device in the form of a radar device, a sonar device or also of an ultrasound device. By means of such a pulse-echo device, a transmitted signal of a very high signal amplitude is radiated via an appropriately embodied transducer into a medium to be examined, in the case of the radar device into the airspace to be monitored, in the case of a sonar device into an area of water to be monitored, and in the case of the ultrasound device into a human body, for example. A received signal being generated by reflection in the medium to be examined is detected and evaluated after having been appropriately amplified. Such a received signal is constituted for example by the signal fed into the amplification device.
In connection with an ultrasound device, a received signal, which is the result of a reflection near the surface of the human body to be examined, has a relatively high signal amplitude. But because of a strong damping by the tissues, a received signal generated by a reflection in a lower tissue layer has a very low signal amplitude. In order to be able to represent an area located deep in the tissue in a satisfactory manner, a good noise behavior is required of an amplifier arrangement. This means that the amplifier arrangement should be constructed to have a low noise output and should be adapted to noise.
Such an amplifier arrangement is known from and described in DE 199 16 902 A1. By setting of the operating point of the semiconductor amplifier employed, which is matched to the expected signal amplitude of the received signal, a mode of operation is made possible which selectively allows an operation optimized as to distortion or optimized as to noise. If a received signal with a high signal amplitude is expected, i.e., a signal reflected at an area close to the surface of the medium to be examined, the disclosed amplifier arrangement operates in a distortion-optimized mode. If, however, a received signal with a low signal amplitude, i.e. a signal reflected at an area located deeper in the medium to be examined, is expected, the disclosed amplifier arrangement operates in a noise-optimized mode. The transition between the two modes of operation takes place by changing the D.C. voltage signals supplied to the semiconductor amplifier for setting the operating point.
A piezo-electric sound transducer is usually employed as the sound transmitter and sound receiver in such an ultrasound device. The electrical equivalent diagram of such a piezo-electric sound transducer which is applicable in case of reception is substantially distinguished by a capacitive resistor located parallel in relation to the sound transducer outlet. For compensating the effects of this parallel capacitive resistor on the frequency response, an amplifier arrangement is presently used, which is described in the technical publication “Piezoxide-Wandler” [piezo-oxide transducers] of J. Koch, 1973, Valvo GmbH Hamburg, on pages 157 and 158, or also in U.S. Pat. No. 4,285,010. In this amplifier arrangement the effects of the parallel capacitive resistor of the sound transducer are at least partially compensated by an inductive resistor switched in series or parallel with the sound transducer output.
Moreover, an amplifier arrangement in the form of an intermediate amplifier for bidirectional signal transmission, having several signal paths, is known from DE 197 54 785 A1. However, the intermediate amplifier does not have a controller for the variable conduct of operations of the amplification elements provided in the intermediate amplifier. The intermediate amplifier is also not intended for pulse-echo operations or for use in an ultrasound device.
Together with the sound transducer, the known amplifier arrangements are sometimes integrated into a single component. This is then identified as an active sound transducer, or active transducer head, since the component contains, besides the actual sound transducer with the amplifier arrangement, also an active component, i.e., one that requires an energy supply. The sound transducer itself is purely passive. It does not require a separate energy supply. Although the integrated amplifier arrangement offers the advantage of signal processing as close as possible to the source, in view of a satisfactory integrating ability, it actually represents an undesired heat source.